A traditional security system consists of a plurality of intrusion sensors located at each secured opening, such as doors and windows. As a rule, the sensors are magnetically operated switches. When the door or window is closed, these switches are held closed. In a supervised system all the switches are connected in series, then connected to a control unit. If one or more switches open or the interconnecting wire is cut, the break in the circuit is detected by the control unit and an alarm condition is reported.
A non-supervised system, which is seldom used, uses switches which are open when the secured door or window is closed. The switches close when the opening is breached. This completes a circuit which is detected by the control unit and an alarm occurs. If the wire to the switches is cut before an intrusion occurs, it is not detected since the circuit simply remains open.
Open types of intrusion sensors such as infrared, microwave, and ultrasonic motion detectors, or photobeams (electric eye), can also be used. Each sensor contains a relay which is energized when no intrusion is occuring. If an intrusion is detected or the internal power supply fails, the relay becomes deenergized and its contacts open breaking the circuit (in a supervised system) to signal an alarm.
One of the problems with prior art systems is the provision of supervision where the area to be protected covers multiple rooms. Wiring the central and remote units of a complete system in such spaces involves substantial expense at installation time and except in commercial warehouse space or the like the wiring must be concealed for aesthetic purposes. The cost of such installations is prohibitive for the individual home or small business and greatly increases the expense for large commercial installations. The attempt to couple the elements of a system by radio link would avoid the problem of wiring but introduces the problem of interference between systems and units of a system since the radio waves transmit through the walls of a building and cause interference within the system or with other radio equipment. The Federal Communications Commission (FCC) in the United States and similar agencies in other countries do not ordinarily permit periodic communication between automatic system elements and thus radio linked systems are incapable of being supervised, i.e., capable of testing their own condition for ready operability in the event of any alarm condition. Ultrasonic systems, on the other hand, have their energy confined by the walls of a room and thus do not present a problem of interference between units which are located in adjoining rooms. This property prevents communication between units of a system which have to cover more than one room. In addition the problem of null conditions for ultrasonic energy is severe because the transmission paths change with environmental parameters such as temperature and humidity such that communication between ultrasonic units cannot be assured at any given time even though no null was experienced when the unit was installed.